At present, it has been growing the concern about attenuating the noise in electric household appliances, automobiles, aircrafts, equipment and machines in general. Said concern results not only from the need to increase the comfort of the consumer, but also due to norms and regulations established by the regulatory agencies defining more severe limits to the admissible noise levels for each type of specific medium.
Sound waves may be absorbed in porous materials and in viscoelastic materials, attenuating the noise levels by absorbing said sound waves.
The use of porous elements for acoustic absorption, manufactured in metallic (or eventually ceramic) materials, tends to be more effective when submitted to higher loads and working temperatures, since said materials have higher mechanical and heat strength than the commonly used elements formed of polymers. Besides, the acoustic muffling elements in metallic and/or ceramic material are easier to be incorporated to traditional mechanical systems.
In engineering, the term “porous materials” is used to designate the materials whose engineering function is made possible by the presence of pores. The pores may be of the primary and secondary type. There are considered primary the pores which are residual, remaining from sintering the powder particles to each other. The size and volumetric percentage of the primary pores result directly from the size of the powder particles used for manufacturing the material and from the processing parameters used (compaction pressure, sintering time and temperature, among others). The referred secondary pores are generated inside the volume of the material, by eliminating the space holders mixed to the matrix powder during the step of preparing the material. In porous materials, the pores may also be classified in pores of the closed type (insulated from the exterior of the material) and pores of the open type (communicating with each other and with the exterior).
The materials which contain only closed pores are applied as a structural support, and the materials with open pores are mainly applied where the passage of fluid is necessary, such as in filtration, catalyst supports, thermal and acoustic insulation, deposition of lubricant (in self-lubricating bushings), and the like. The particular process, used to produce the porous materials, defines the properties and porous structure of said materials, such as type of porosity (open or closed), volumetric percentage of the pores, dimension and shape of the pores, uniformity and connectivity of the pores.
Porous materials, with open porosity, may be manufactured by the processing routes, such as replica, deposition of material (INCOFOAM), or by mixing a two-phase composite, constituted by a continuous matrix of homogeneously dispersed metallic or ceramic particles, with a space holder, or also by the rapid prototyping technique. Materials with closed pores may be produced by combining a metallic or ceramic matrix with hollow spheres (“synthetic foams”), compacting the alloy powder mixtures with foaming agents, sintering the loose powder (not compacted) within a die, or with a material in liquid state, by injecting gas directly in the liquid-state metal or by adding a pore forming agent.
Several alternative processing methods for the production of porous materials have been proposed over the years. However, for the particular application intended in the present invention, that is, the acoustic absorption in equipment or machines such as, for example, hermetic compressors, the finished porous components should have low cost, should be produced in large scale and by a cost-effective and high productivity process. Furthermore, the raw-material used should have a low cost. Therefore, the powder metallurgy is presented as a process for manufacturing finished components with great potential. However, the high open porosity, which is required for the porous body of the acoustic muffling element, cannot be achieved only by adjusting the parameters of the powder manufacturing process, such as compaction pressure, sintering time and temperature, which parameters are related only to obtain the primary pores. In order to achieve a high percentage of porosity, comprising secondary open pores, besides the primary pores eventually opened (communicating with each other and with the exterior), it is necessary to add a space holder to the particulate material composition to be sintered.
For the development of porous bodies with an efficient acoustic absorption, it should be considered their behavior regarding the degree of acoustic absorption resulting from the morphological characteristics of the porous structure of the muffling element, which behavior may be predicted through simulations by using analytical models of acoustic propagation as, for example, the Zwikker/Wilson model for the acoustic propagation in porous materials presenting a rigid structure. For the exemplary application in the discharge of a hermetic refrigeration compressor, the simulations indicate that the highest coefficient of acoustic absorption in metallic porous bodies occurs, for a volumetric percentage of communicating pores (open) between 45% and 60%, when the interconnected pores present a diameter between 20 μm and 60 μm. Said information was relevant as a starting point to carry out the experimental development of the material of the present invention, aiming at obtaining a porous structure theoretically more appropriate for the acoustic absorption.
As mentioned above, for the exemplary application in hermetic compressors, among all the techniques for manufacturing porous materials, the one which shows a high potential for a cost-effective scale production of the porous elements, having the specified porous structure, is the technique of powder metallurgy. The powder metallurgy presents a wide variety of different techniques for the formation or consolidation of the “feedstock” in a finished or semi-finished component: uniaxial compaction in matrices, isostatic compaction, rolling, extrusion and injection of powders, barbotine gluing, and others. The compaction process, via uniaxial pressing in a matrix, is considered the most appropriate, for presenting, as main characteristic, the feasibility of a cost-effective serial production of elements (pieces) with final dimensions and geometry, since the process can be easily controlled and automated, further allowing the desired microporous structure to be easily produced, by mixing the space holder, in the form of powder, to the metallic or ceramic matrix powder.
The manufacture of metallic porous elements, formed by powder metallurgy and presenting open primary pores and open secondary pores, uses raw material in the form of metallic powders as a matrix phase, instead of ceramic powders, which is used by the present invention, as discussed ahead. However, the metal powders, especially when required to be very fine, as in the present case, are costly, making economically unfeasible to obtain the porous components of low cost for application in commodities.
Considering the need, in almost all applications, to form a rigid metallic porous element, the material commonly used in the known prior art techniques is defined by a fine metallic material powder as, for example, a powder of iron, copper, nickel, molybdenum, tungsten, cobalt and mixtures thereof, having a known very high cost and, thus, generally not economically interesting.